Multiple ion channels influence neuronal excitability, and these are often subject to modulation by neurotransmitters. Prominent among these are background K+ channels that are targeted for inhibition by neurotransmitters, leading to membrane depolarization and increased excitability. G protein-coupled receptors capable of mediating this effect have been identified for many transmitters (invariably those that couple via Gaq/n-family subunits), and it represents a predominant mechanism for slow synaptic excitation throughout the brain. The molecular identity of background K+ channels targeted for inhibition are unknown in most native systems, and the mechanisms of receptor-mediated channel inhibition remain obscure. Proposed research explores novel mechanisms and molecular substrates underlying Gaq-linked inhibition of background K+ channels. Our studies of cloned two-pore-domain background K+(K2P)channels - TASK-i (K2Ps) andTASK-3 (K2Pg) - has revealed a novel mechanismfor Gaq-mediated ion channel modulation. We find that TASK channel inhibition is independent of phospholipase C (PLC) activation and PI(4,5)P2 depletion, but instead requires Gaq interaction with the channels or with a closely-associated intermediary. Wepropose studies designed to identify molecular determinants that accountfor Gaq association and TASK channel inhibition, and to examine if this PLC- independent mechanism contributes to inhibition of other types of background K+ channels and their neuronal correlates by Gaq. Our published and preliminary work has identified TASK channels as substrates for background K+ currents in cholinergic neurons, specifically motoneurons and striatal interneurons, based on a constellation ofvoltage-dependent and pharmacological properties. This tentative identification requires verification. We propose to use newly available knockout mice to test definitively the TASKsubunit contributions to these native neuronal backgroundK+currents. Interesting preliminary data indicates that TASK currents are not targets for Gaq-mediated inhibition in striatal cholinergic interneurons. Rather, a novel Ch-activated background K+ channel is inhibited by Gaq-linked metabotropic receptors (mGluRs). Wepropose experiments to determine if the recently identified Slo2 channels account for this mGluR-sensitive channel, and to identify the relevant Gaq-mediated inhibitory mechanism. The following Specific Aims are proposed: [i] Establish mechanisms underlying PLC-independent modulation ofTASK and GIRK channels by Gaq subunits; [2] Identify background K+ channels in striatal cholinergic interneurons and elucidate mechanisms that contribute to Gaq-mediated activation. These experiments will characterize molecular substrates underlying native neuronal neurotransmitter-modulated background K+ currents and examine molecular mechanisms by which they are modulated.